1. Field of the Invention
This invention relates to a selectively permeable film and an ion sensor. More particularly, this invention relates to a selectively permeable film having a polymer film suitable for use such as an ion sensor applied directly in the form of a coat to the surface of an electroconductive substrate and to an ion sensor using such a selectively permeable film. Particularly, the ion sensor contemplated by this invention is of a type having a polymer film applied directly in the form of a coat to the surface of an electroconductive substrate and serving the purpose of permitting the measurement of an ion concentration in a given solution by means of electrode potential or current response.
2. Description of Prior Arts
Recently, the coating on the surface of an electroconductive material with a film for the purpose of conferring various functions upon the electroconductive material has been gaining in popularity. Generally, an electrode coated with such a film is called a polymer coated electrode. The coated electrode is expected to serve as an electrode capable of manifesting a novel function suitable for use in catalysts, transducers and sensors. For the exploitation of the function, due consideration should be paid to the quality of the electroconductivity and ion-selectivity of the film applied directly in the form of a coat to the surface of the electroconductive substrate. The electrodes of the class such as are used in the form of ion sensors and pH sensors, for example, are required to possess selectivity and permeability with respect to the kinds of ions subjected to detection. No film material which possesses the selective permeability and, at the same time, functions advantageously as a coat for the electrode, however, has so far been found.
Heretofore, the hydrogen electrode and the quinhydrone electrode have been known as electrodes available for measuring the concentration of a hydrogen ion in a solution. Nowadays, because of simplicity of usage and improved accuracy of measurement, the glass electrode has come to find increasing acceptance. The principle which underlies the measurement of the pH value of a given solution by the glass electrode is that when the given solution of an unknown hydrogen ion concentration and the reference solution of a constant hydrogen ion concentration are opposed to each other across a thin glass membrane, the potential difference between the two solutions appears on the opposite surfaces of the thin glass membrane and the measurement of this potential difference permits determination of the pH value.
The glass electrode, therefore, is required to possess an area for holding the reference solution and does not permit easy reduction of size. When the given solution happens to contain a viscous substance, the viscous substance adheres to the surface of the thin glass membrane rendering the measurement of the pH value difficult or impairing the reproducibility of the electrode potential response. Further, since the resistance offered by the glass membrane of the glass electrode is so large as to range from 10 to 100 M.OMEGA., the ordinary potentiometer cannot be used by itself but must be used in combination with an amplifier of high-input impedance for effective measurement of the pH value.
An object of this invention, therefore, is to provide a selectively permeable film which is capable of being directly coated on the surface of an electroconductive substrate and, at the same time, is possessed of ion-selective permeability.
Another object of this invention is to provide an ion sensor which obviates the necessity for providing an area for holding the reference solution and permits desired reduction in size.